Make the first Stats tab a default one if there's none selected

[staskus]

Fixes #

Make the first Stats tab a default one if there's none selected.

The issue with the current code is that user defaults return 0 even if integer value doesn't exist. 0 is the raw value of Insights tab.

To test:

Feature flag enabled

1. Fresh install Jetpack app
2. Enable Stats Subscribers and Traffic tab feature flag
3. Open Stats
4. Confirm Traffic is selected by default
5. Change to Subscribers
6. Leave and come back
7. Confirm Subscribers is selected

Feature flag disabled

1. Fresh install Jetpack app
2. Disable Stats Subscribers and Traffic tab feature flag
3. Open Stats
4. Confirm Insights is selected by default
5. Change to Years
6. Leave and come back
7. Confirm Years is selected

Regression Notes

1. Potential unintended areas of impact

None

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23167-43973c0
	Version	24.8
	Bundle ID	org.wordpress.alpha
	Commit	43973c0
	App Center Build	WPiOS - One-Offs #9830

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23167-43973c0
	Version	24.8
	Bundle ID	com.jetpack.alpha
	Commit	43973c0
	App Center Build	jetpack-installable-builds #8878

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[guarani]

Thanks @staskus! Looks good, left a nitpick.